a) Field of the Invention
This invention relates to a remaining capacity meter and detection method for an electric vehicle battery, which are suited for the prediction of the remaining capacity of a battery whose fully-charged capacity deteriorates at varied rates, like a nickel-base battery.
b) Description of the Related Art
In recent years, there is an ever-increasing move toward protection of the global environment. The air pollution due to the mass consumption of fossil fuels, in particular, has become a serious problem. Prevention of the air pollution is an extremely important theme for the protection of the global environment.
In automotive vehicles, engines making use of a fossil fuel such as gasoline or diesel fuel are also dominant these days. A substantial progress has been made for the purification of exhaust gas from these engines. Electric vehicles, on the other hand, are now receiving increasing attention because they do not emit exhaust gas by themselves.
At this stage electric vehicles are however still accompanied with various practical problems to be overcome. Electric vehicles have therefore not spread to the public although some electric vehicles have found utility in certain limited fields. To make electric vehicles more practical, a variety of techniques have been proposed recently with respect to electric vehicles.
As an important element of an electric vehicle, there is a battery (hereinafter called the "main battery"). The remaining capacity of the main battery is equivalent to remaining fuel, so that it is desired for a driver to be always aware of this remaining capacity.
Remaining capacity meters for electric vehicle batteries (hereinafter called the "remaining capacity meters") have therefore been developed to determine and indicate the residual capacity of such an electric vehicle battery.
Correlation is known to generally exist between the remaining capacity of a battery and the open end voltage of the battery. Devices for determining the remaining capacity of a battery by using this characteristic have also been proposed.
For example, the battery monitoring device disclosed in Japanese Utility Model Publication (Kokoku) No. HEI 3-33121 makes use of the correlation between the remaining capacity of a battery and the open end voltage of the battery. This device detects a stable open terminal voltage of the battery when the battery is not discharged, and then stores the value of the stable open terminal voltage. Upon discharge of the battery, the device stores the value of an open terminal voltage when the open terminal voltage has become stable again subsequent to end of the discharge. The stored value is updated in this manner so that the device displays the value so updated. This battery monitoring device performs the updating for the reasons to be described next.
When a battery is repeatedly discharged, the remaining capacity of the battery changes as indicated by a dashed line X in FIG. 7. As also shown in FIG. 7, the open terminal voltage of the battery, however, changes significantly as indicated by a solid line so that, although the open terminal voltage corresponds to the remaining capacity of the battery while no discharge takes place, the open terminal voltage abruptly drops upon discharge and no longer corresponds to the remaining capacity of the battery. The remaining capacity is therefore not indicated correctly if the detected value of the open terminal voltage is always shown as a quantity corresponding to the remaining capacity of the battery. For this reason, the open terminal voltage corresponding to the remaining capacity of the battery before discharge is displayed during the discharge.
The battery monitoring device is constructed as shown in FIG. 8. Shown in FIG. 8 are a battery 101, a load 102, resistors 103,104, an LED-lighting linear IC 105, a reverse-current preventing element 106, capacitors 107,110, resistors 108,111, a transistor 109, an LED 114, a voltage detection unit 115, a voltage register unit 116, a display unit 117, and a reset/correction unit 118.
At the voltage detection unit 115, an open terminal voltage of the battery 101 is divided by the resistors 103,104. A voltage obtained through the resistor 104 charges the capacitor 107 in the voltage register unit 116 so that the open end voltage of the battery 101 is stored as a voltage charged in the capacitor 107. According to the voltage so charged in the capacitor 107, the LED 114 of the display unit 117 is lit via the linear IC 105.
At the reset/correction unit 118, during use of the battery 101, no voltage appears across the resistor 111 so that the transistor 109 remains off. The capacitor is therefore not discharged by an external circuit so that, for example, a voltage A in FIG. 7 is shown at the display unit 117.
When the use of the battery 101 is interrupted, the voltage of the battery 101 arises. This rise is detected by the voltage detection unit 115 and a charging current flows into the capacitor 110 of the reset/correction unit 118. An induced voltage therefore appears between opposite ends of the resistor 111 to turn on the transistor 109. Via the resistor 108 of the reset/correction unit 118, the capacitor 107 is discharged so that the voltage of the capacitor 107 drops. At the same time, the open end voltage of the battery 101 rises and, when the voltage of the capacitor 107 has become equivalent to the open end terminal voltage of the battery 101, the discharge of the capacitor 107 stops. Then, the voltage of the capacitor 107 becomes equal to the open terminal voltage of the battery 101.
As a consequence, the voltage charged as a stored value in the capacitor 107 changes similarly to the remaining capacity X of the battery as indicated by dashed lines A.fwdarw.C.fwdarw.D.fwdarw.B in FIG. 7. In accordance with the voltage charged in the capacitor 107, the remaining capacity is shown at the display unit 117.
Further, a method for detecting and indicating the capacity and service life of a battery for an automotive vehicle is proposed in Japanese Patent Application Laid-Open (Kokai) No. SHO 60-245402. When either the open terminal voltage of a battery or the battery voltage during running has dropped below its predetermined value, this drop is interpreted as a decrease in the capacity of the battery and an alarm is produced. When the battery voltage has arisen beyond a predetermined charging voltage in a predetermined time while the battery is being charged for a reduction in battery load, an increase in engine speed or the like shortly after the production of the alarm, an alarm is produced to indicate that the battery is in a final stage of its service life. When the battery voltage conversely does not exceed the predetermined charging voltage within the predetermined time, an alarm is produced to indicate the need for charging. When the battery voltage has reached the predetermined charging voltage when the predetermined time has elapsed or later, the battery is indicated to be in order.
Upon charging a battery, on the other hand, it is possible to charge it to a maximum capacity, namely, a full-charge capacity by charging it for a sufficient time. As a conventional method for the determination of the remaining capacity, it is commonly practiced to add or subtract the integral, with respect to time, of a current discharged from or charged into the battery to or from its capacity at the time of full charge.
Incidentally, such an electric vehicle battery is discharged as a result of a supply of electric power to a motor. The battery is also discharged by a self-discharge and/or a dark current even while the battery is not used during stoppage of the motor. Upon determination of the remaining capacity, it is therefore necessary to take such a self-discharge and dark current into consideration.
In particular, the self-discharge is also dependent on a left-over time of the battery and the level of temperature of the battery. Devices have hence been proposed to determine the remaining capacity of a battery by predicting the quantity of a current discharged as a dark current and also estimating the quantity of a self-discharged current while taking the temperature of the battery into consideration.
Such devices include, for example, the remaining battery capacity meter disclosed in Japanese Utility Model Application Laid-Open (Kokai) No. SHO 61-102976. This remaining capacity meter predicts the capacity of the battery remaining at a given time of prediction. The prediction of the remaining capacity is performed as shown in FIG. 9.
Namely, a measurement value of a battery capacity sensor is read (step 31), a remaining capacity C.sub.0 of the battery is calculated (step 32), an inputted time Y.sub.0 of the measurement by the battery capacity sensor is read (step 33), and an inputted time Y.sub.1 for prediction of the battery capacity is read (step 34). A dark current consumption C.sub.1 is then calculated based on the time until the time Y.sub.1 for prediction (i.e., Y.sub.1 -Y.sub.0) (step 35).
Stored temperature data from the measurement time Y.sub.0 till the prediction time Y.sub.1 are then read (step 36), and stored self-discharged current data corresponding to the temperatures are read (step 37). The term "self-discharged current data" as used herein means temperature-dependent data of remaining capacity percentages as a function of the number of days during which a battery was left over as shown in FIG. 10. A self-discharged current quantity C.sub.2 is calculated from such self-discharged current data (step 38). Further, the stored data of temperature at the prediction time Y.sub.1 is also read (step 39) and, based on the temperature data at the prediction time Y.sub.1, a change rate C.sub.3 of the remaining capacity for the temperature is calculated (step 40).
Based on these calculated values C.sub.0, C.sub.1, C.sub.2 and C.sub.3, the remaining capacity C[=(C.sub.0 -C.sub.1).multidot.C.sub.2 .multidot.C.sub.3 ] is calculated (step 41) and is displayed (step 42).
In addition, a system for switching charging methods by taking the temperature history of a battery as an additional factor for determination is proposed in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-355632.
A device is also disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 5-276603. According to this device, the quantity of a charged or discharged current is determined from a charged or discharged current which has been detected. This charged or discharged current quantity is added to or subtracted from a value of a remaining capacity memory to determine the remaining capacity. A charging or discharged current is corrected based on a capacity reduction characteristic inherent to a battery especially when the charging or discharged current is equal to or greater than a rated current.
In general, a battery is gradually deteriorated while it is repeatedly charged and discharged so that, as a characteristic thereof, its full-charge-time capacity itself also decreases gradually. Although a battery is generally considered to deteriorate depending on the number of such charge-discharge cycles, nickel-based batteries which are employed as batteries for electric vehicles deteriorate at varied rates in such full-charge-time capacity, thereby making it difficult to accurately predict the remaining capacities of the batteries.
In the case of a nickel-based battery, this is attributed to the fact that the deterioration of the battery varies depending on its temperature. For example, FIG. 4 diagrammatically illustrates deteriorations of a chargeable nickel-cadmium battery (Ni-Cd battery), a nickel-based battery, in full-charge-time capacity as a function of the number of charge-discharge cycles for different battery temperatures. As is shown in the diagram, it is appreciated that the deterioration becomes severer as the temperature of the battery goes up.
Although the above-described conventional techniques include those determining the remaining capacity of a battery while taking the temperature of the battery into consideration, for example, like the meter disclosed in Japanese Utility Model Application Laid-Open (Kokai) No. SHO 61-102976, none of them take the number of charge-discharge cycles of the battery into additional consideration so that variations in the deterioration of the full-charge-time capacity are not specifically taken into account. It is therefore difficult to accurately predict the remaining capacity of a battery.